# Thread: Calculating heatloss caused by vent ?

1. New Guest
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## Calculating heatloss caused by vent ?

We've got a property where we have done the heatloss calcs, to size the new radiators.

In most cases we are 20% over the target BTU output, so we have a bit of margin.

The project has been instructed to install vents in some rooms for gas burning fires and cookers. (UK building regs)

How much heatloss would a 100x100mm constantly open to atmosphere vent cause ?

Based upon a) an external temp of 10C, internal 20C. b) an external temp of 2C, internal of 20C.

2. you can not calculate the heat loss without knowing the cfm going through the vent and there is a spot in manual J for air infiltration and exhaust vent but a relatively smal vent like that should not affect the calculation too much.

3. Once you know the air flow, convert to CFM then it's CFM * i.08 (sea level) * delta T the temperature difference. So you can see the load will be different depending on the temperature difference.
Depending on how the air flow is measured makes a difference also. If measured as velocity, the grills "K" factor ( inherent resistance)will need to be known to convert velocity to CFM. Grill size * K gives the free area, then velocity * area = CFM.
There might be warmer days where the vent won't add to the heat load but subtract from it.

4. At 20°C inside and 2°C outside. 345.6 BTUs per 10 CFM/.28CMM.

5. Professional Member
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With a 20% cushion I would worry more about the location of the vent than the CFM. Locate the vent as close to the flame source as possible and to prevent noticeable drafts on the occupants. If properly sized the vent should supply combustion air and eliminate infiltration.

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